tion and output reduction may be proportional to each other. One must add, however, that even if it were known that this relationship is true for agriculture in cases where considerable disguised unemployment, or observed unemployment, exists, the reduction in physical capacity per man need not result in an actual reduction in total output. It may simply mean that more of the unemployed become absorbed in the work involved.
The economic consequences of all these effects are not entirely clear. Both the sib-number and sib-spacing effects seem to diminish physical size, linguistic skills, relative immunity from disease, and I.Q. (17, p. 130). It should be emphasized that these elements are not entirely separate from one another. Nurture effects will also affect the consequences of formal schooling in the sense that the capacity to absorb formal schooling will depend to a great degree on the nurture aspects.* Thus what appears as part of the economic returns to formal schooling is in fact a return to nurture, since it is the nurture elements that determine the capacity to take advantage of formal schooling. The main element to be noted is that a greater rate of population growth will set in motion demographic causative effects all of which have an adverse impact on economic growth.
The argument presented is that higher rates of population growth (compared to lower rates) are associated with (a) a younger population and hence a higher dependency ratio, (b) usually a higher average sib number, (c) usually closer sib spacing, and probably (d) a greater number of pregnancies per woman and perhaps higher maternal mortality and morbidity. (This last depends upon the degree to which the higher rate of population growth is a result of lower mortality rather than higher fertility.) The impacts of these four demographic effects on dependency, malnutrition, degree of maternal deprivation, speech and personality formation, I.Q., and on success indicators are all adverse to economic growth and the average acquired economic quality of the labor force.
For the most part the data are only suggestive, providing clues to the importance of the elements considered. Unfortunately, there is a lack of statistical information as to how important these elements are from a macro-economic viewpoint. We do not know what rates of population growth at what level of per capita income will lead to what degree of malnutrition or
*Tanncr (16, p. 211) states that Doughs in 1960 reported from a sample of children in Great Britain that "early maturers had gained significantly more successes than late maturcrs in the examination for entry to secondary schools at age 11. Not only were their test papers better; the reports of the teachers upon their behavior in class also favored them." However, the later maturers catch up when they reach their physical growth spurt.